When a human knee is bent or the body is turned, a joint surface of the knee between the femur and the tibia performs a rotational motion in an axial direction, or performs a translational motion on abduction or adduction, in the sagittal or coronal plane, and the above mechanism of knee motion is to absorb or control the force generated during exercise with the help of ligaments and muscles, to support the load and help perform more natural exercise.
However, in the case where it is hard to perform a motion by the natural knee joint due to arthritis caused by aging or various diseases, and knee damage caused by external impact, surgical procedures of implanting an artificial knee joint can be used to achieve the function according to the above mechanism of knee motion.
Generally, the artificial knee joint is roughly constituted by a thighbone coupling member, a tibia coupling member, a bearing member, and a knee coupling member, and it can take various forms in structure, material, surface treatment process, etc., of which a PS type artificial knee joint that is used when a posterior cruciate ligament is removed is configured such that the bearing member includes a post functioning as the posterior cruciate ligament, and the thighbone coupling member includes cam allowing smooth rotation by being engaged with the post, wherein based on the post of the bearing member, the direction in which the knee coupling member is located is defined as anterior and the direction in which the cam of the thighbone coupling member is engaged with the post is defined as the posterior of the artificial knee joint, and in the following description of the present invention and drawings, if the front and rear are to be indicated, they should be denoted by A and P respectively.
Further, in the process of performing a motion after the surgery of the PS type artificial knee joint, when the thighbone coupling member is severely bent backward by a rollback motion in which the femur bends backward, a gap between the post and the cam that were in contact with each other is increased and may not be restored, resulting in eventual dislocation. When the dislocation of the thighbone coupling member occurs, the artificial knee joint is unable to perform normally until it is re-operated on, and thus preventing dislocation is a very important task that must be achieved in the artificial knee joint field.
Further, in relation to the problem of the dislocation of a thighbone coupling member, when dislocation begins to occur, a maximum gap of the post and cam is referred to as a jump distance, and to prevent dislocation, it is advantageous to increase the jump distance as much as possible, but simply increasing the height of the post may result in increased wear of the bearing member or increased amount of bone to be cut in knee replacement surgery. Hereinbelow, the problem of the conventional artificial knee joint will be described in detail with reference to FIGS. 1 and 2.
FIG. 1 shows a conventional artificial knee joint, wherein as shown in FIG. 1(a), a post 31a is configured such that an upper surface is not inclined, and a posterior vertical surface is formed to be linear, so a jump distance D1 is low, whereby as shown in FIG. 1(b), it has a structure in which it is difficult to prevent dislocation of the thighbone coupling member 1, and since the upper surface of the post 31b is in a horizontal plane, it is difficult for the thighbone coupling member to be restored to a normal state after it is dislocated. In FIG. 1(c), a dark shaded area below a dotted line indicates the amount of bone to be cut in knee replacement surgery.
FIG. 2 shows the case of simply increasing the height of the post in the conventional artificial knee joint of FIG. 1, wherein as shown in FIG. 2(a), the basic shape of a post 31b is the same as the conventional artificial knee joint of FIG. 1, but by increasing the height of the post, a jump distance D2 is larger than the jump distance D1 of the conventional artificial knee joint.
Accordingly, as shown in FIG. 2(b), the dislocation prevention effect of the thighbone coupling member is improved compared to the conventional artificial knee joint of FIG. 1, but as shown in FIG. 2(c), when a cam 13 of a thighbone coupling member 1 is dislocated, it is difficult for the thighbone coupling member to be restored to a normal state since an upper surface of the post 31b is formed in a horizontal plane.
Further, the amount of bone to be cut in knee replacement surgery is further increased by a dark shaded area shown in FIG. 2(d) compared to the amount of bone to be cut shown in FIG. 1(c).
Thereby, to solve the above problem of dislocation of the thighbone coupling member, a stop member is provided inside the thighbone coupling member, or the cam is placed higher from anterior toward upward.